A standard self-closing damper assembly for a duct has a support adapted to be secured in the duct and having a pivot defining a pivot axis traversing the duct, a pair of damper flaps pivotal on this pivot about the axis between an open position extending in line with the duct and a closed position generally coplanar transverse to the duct. Thus the flaps define in the open position open flow spaces to both sides of themselves and in the closed position the flaps substantially block flow through the duct. One or more springs urge the damper flaps continuously into the closed position. A fusible link is connected between the flaps offset from the axes to retain the flaps in the open position unless the link is heated beyond a predetermined temperature.
Thus when the link melts the springs snap the flaps into the closed position, thereby effectively blocking off flow in the duct. The entire assembly can be made at low cost of sheet metal and is required in fire-resistant construction between the so-called fire cells. When closed, such a damper assembly is effective to block flow in one direction like a standard flap-type check valve, and in the opposite direction its spring is normally designed to exert a very great force to hold it closed.
This type of construction has some drawbacks. It constitutes a flow restriction in the open position in which the two flaps are back to back in the center of the arrangement with the spring, pivot rod, and link between them. Such flow restriction must be designed for in carefully calculated work, perceptibly increasing overall system costs for ductwork of larger flow section, more powerful blowers, and so on. Furthermore, although effective to block flow in both directions for the reasons given above, such a damper assembly must be mounted relative to the normal direction of flow through the duct with the link lying upstream of the pivot axis, so that the link is not shielded by these parts from the heat in the duct. If mounted backward, the response time is increased considerably. This decreased sensitivity to flow in one direction is also disadvantageous, as during a fire flow can easily reverse in a heating or air-conditioning duct, so that the nominal rating (closing temperature of about 160.degree. F.) is confused by different response times for flow in opposite directions. The slower rating is always employed, so that occasionally such damper assemblies close prematurely.